The brain develops in a local (internal) environment that is distinct from the rest of the embryo, but is also different from the adult. The composition of the fluids (brain extracellular fluid, ECF and cerebrospinal fluid, CSF) that form the internal environment of brain is controlled by mechanisms referred to collectively as "the blood-brain barrier". Recent work of the Pl's group suggests the route of entry from blood to brain via the CSF in the developing brain may be more important than a direct route across blood vessels, which in immature brain are few. The long term aim is to understand the nature of mechanisms that control the composition of the internal environment of developing brain and to determine properties of its contribution to specific features of brain development. An additional aim is to understand effects on brain development resulting from pathophysiological disturbances to barrier mechanisms. The proposals concentrate on two aspects of control of the developing brain's internal environment, one a normal property of brain barrier mechanisms and the other, pathophysiological changes in barriers properties due to an inflammatory response:(i) Definition of permeability mechanisms and pathways between blood, CSF and brain at very early stages of development. (ii) Studies of acute & long-term effects of simulated infectious inflammatory responses (evoked by injection of lipopolysaccharide, LPS) on brain barrier permeability at different stages of development and in adults. The studies will be in S. American opossums (Monodelphis domestica); they are born at a very early stage of brain development (equivalent to 13 day fetal rats). The permeability mechanisms and pathways to be studied are to proteins and small molecular weight (mw) polar molecules. Biotin (244 Da), biotin labelled probes (eg biotin dextran mw 3000Da) and proteins (albumin & fetal protein, fetuin) will be injected intraperitoneally at different postnatal (P) ages up P65 (postweaning young adult) and their distribution in CSF and brain examined by light and electronmicrosopy. Their transfer into CSF will be quantitated and compared with results with classical but non-visualizable permeability probes (radiolabelled sucrose & inulin). Similar experiments, will study the effects of an acute or chronic inflammatory response (induced by injection of LPS for different periods) on barrier permeability at different ages.